Field of the Invention
The present invention relates to a turbine engine rotor wheel.
Description of the Related Art
A rotor wheel conventionally comprises a disk carrying fan blades at its outer periphery, these blades having roots that are engaged in substantially axial slots in the outer periphery of the disk. The fan blades are held radially on the disk by co-operation between the shapes of their roots and of the slots in the disk, the blade roots being of the dovetail type, for example. Inter-blade platforms are mounted on the disk between the fan blades or they form integral portions of the disk. Blades of this type are described for example in document FR 2 881 174, in the name of the Applicant. The slots, which may also be referred to as sockets, may be straight or curvilinear, and the contact surfaces between the roots of the blades and the inside walls of the slots are referred to as bearing surfaces.
The blades are mounted with clearance between their roots and the walls of the slots. In order to reduce this clearance, proposals have already been made to mount resilient pressers between the bottoms of the slots and the blade roots, so that the blade roots are held pressed radially against the bearing surfaces of the disk. Nevertheless, the use of pressers is possible only with blades of large size, such as fan blades, since large size makes it possible to fabricate the pressers and for them to have sufficient stiffness to be capable of being mounted by force without being deformed.
The pressers must also be very accurate so that the blades are held, but without being completely blocked in a position that does not enable the blades roots to press normally against the bearing surfaces of the slots.
That type of rotor wheel presents the drawbacks set out below.
Rotation of the rotor wheel during operation of the turbine engine causes high levels of radial force to be applied to the blades, thereby keeping them in their operating or “working” positions. By way of example, a high pressure compressor blade weighing 17 grams (g) may be subjected to a centrifugal force of about 500 kilograms (kg), and a fan blade weighing about 4.5 kg may be subjected to a force greater than 60 (metric) tonnes (t).
In the absence of a presser or when the pressers are not sufficiently prestressed, every time the rotor wheel is stopped, it takes up a different angular position and the blades change position under the effect of their own weight.
Balancing the rotor wheel consists in eliminating any unbalance in the wheel, and this is performed at low speed, however since the positions of the blades, and thus the unbalance of the rotor wheel, are different on each occasion the wheel is started, it is not possible for this balancing to be performed with sufficient accuracy.
In addition, the small-amplitude movements under low loading of the blades that occur each time the turbine engine is started, give rise to wear on the inside surfaces of the slots for mounting the blade roots. This is known as “fretting” wear, and it can prevent the integrity of the disk and of the blades being inspected by using eddy currents, and that can lead to a disk and/or blades being scrapped even though they would normally still be usable.
In a variant, the blades may be of the type having a hammerhead attachment, the blade roots then being mounted in a circumferential slot. In the same manner as above, the blades are held radially on the disk by co-operation between the shapes of their roots and of the circumferential slot in the disk. By way of example, blades of that type are described in document FR 2 900 989, in the name of the Applicant.
The blade roots are inserted radially into the circumferential slot through a single notch in the disk, with the blades then being slid circumferentially along the slot to their final positions. The blades are held by a latch, with circumferential clearance being maintained between the blades in order to accommodate thermal expansion during operation of the turbine engine.
It is not possible to mount pressers between the blade roots and the bottom of the circumferential slot.
Once the blades are mounted they are movable along the slot over a distance corresponding to the total value of the circumferential clearances between the blades.
Thus, each time the wheel is stopped, the blades can change position since the angular position of the wheel is not necessarily the same. The blades then become repositioned in random manner under their own weight within the limit of the available clearance. These changes in the positions of the blades have several direct consequences on the balancing of the rotor wheel and on its lifetime.
At the time of balancing, the changes in the positions of the blades lead to dispersion in measurements of the unbalance from one engine start to another. This dispersion puts a limit on the accuracy with which balancing can be performed, and that has significant consequences on the forces transmitted to the bearings and on the levels of vibration that are perceived.
Another consequence is lateral wear of the platforms, which press against one another with relative movement on each start. This wear further amplifies the tangential clearance between the blades, thereby leading to greater unbalance and to wear of the bearing surfaces of the disk and of the blade roots.
In the same manner as above, the movement of the blades can also give rise to fretting wear of the inner walls of the blade root mounting slots, thereby making it difficult to check them for cracks by using eddy currents.
Finally, there is the possibility of using bladed disks that are made as a single piece, in which the blades and the disks are made as a single piece by machining.
Nevertheless, that technique cannot always be applied to all of the rotor wheels, in particular because of thermal stresses, of mechanical stresses, and of weight stresses, and that technique presents very high costs in terms of manufacturing and maintenance.
It is also difficult if not impossible to machine cooling circuits in the blades of single-piece bladed disks or “blisks”.
Documents JP 2007-120460, DE 32 36 021, JP 1-237304, and JP 61-129405 describe using shape memory pressers that are suitable for deforming between two positions as a function of temperature.
The cooling and the heating of those pressers is relatively difficult to implement and to control.